The doubly-fed induction machine has several advantages over conventional induction machines in various applications such as generators—for example, in Wind Turbine Generators (WTGs), motors—for example, as electric drives in vehicles, or as a combination of a generator and a motor—for example in flywheel emergency power generators. Firstly, since the rotor excitation is controlled by the VCS, the doubly-fed induction machine is able to both import and export reactive power enabling the doubly-fed induction machine to remain connected to the electric grid during substantial voltage disturbances. Secondly, the control of the rotor excitation enables the doubly-fed induction machine to remain synchronized with the electric grid while the rotor—for example connected to a wind turbine—varies in speed. A variable speed wind turbine utilizes the available wind resource more efficiently than a fixed speed wind turbine.
The decoupled P-Q control of a doubly-fed induction machine requires information about the position of the rotor winding axes with respect to the stator winding axes. Decoupled P-Q control techniques absent mechanical sensors as disclosed, for example, in:    L. Xu and W. Cheng, “Torque and reactive power control of a double fed induction machine by position sensorless scheme”, IEEE Trans. Ind. Applicant., Vol. 31 pp. 636-642, May/June 1995;    R. Pena, J. C. Clare, and G. M. Asher, “Doubly-fed induction generator using back-to-back PWM converters and its application to variable-speed wind-energy generation”, Proc. Inst. Elect. Eng., Pt. B, Vol. 143 No. 3, pp. 231-241, May 1996;    L. Morel, H. Godfroid, A. Miraian, and J. M. Kauffmann, “Doubly-fed induction machine: converter optimization and field oriented control without position sensor”, Proc. Inst. Elect. Eng., Pt. B, Vol. 145, No. 4, pp. 360-368, July 1998; and,    H. Akagi and H. Sato, “Control an performance of a Douly-Fed Induction Machine intended for a flywheel energy storage system”, IEEE Trans. On Power Elect., Vol. 17, No. 1, January 2002;are based on determining the rotor position from knowledge of numerous parameters of the doubly-fed induction machine and information of the instantaneous voltages and currents. A major disadvantage of these methods is that they are dependent on machine parameters that change over a time period of machine operation such as, for example, changing rotor resistances due to wear in the brush slip ring contact.
It would be highly desirable to overcome these drawbacks and to provide a method and system for decoupled P-Q control absent mechanical sensors that is independent from changing machine parameters.